Utilization of the sensor effect of the piezoelectric actuator for measuring the frequency of a pressure wave, which is generated by the opening and closing of the nozzles, is described in published German patent document DE 102 17 592, for example. The piezoelectric actuator is used to open and close the control valve of the fuel injector in order to control the injection operation. The fact that the piezoelectric actuator is able to convert electric voltage into force and electric charge into linear expansion is utilized for this purpose. The reversal of these effects is utilized to convert the mechanical force exerted on the piezoelectric actuator into an electrical voltage signal. This is known as the sensor effect.